1. Field of the Invention
The invention relates to an etching solution for etching away a metal layer from a substrate which includes a hydrogen containing compound that dissolves the metal per se while developing hydrogen, and a nitrosubstituted organic compound, to a method for etching away a metal layer from a substrate wherein the substrate coated with the metal is immersed into an etching solution which contains a hydrogen containing compound that dissolves the metal per se while developing hydrogen, and includes a nitro substituted organic compound, preamble of the eighth claim and to the use of a water soluble, easily hydratable organic nitro compound as an addition to the aqueous solution of an acid or a highly basic metal hydroxide in order to prevent the release of hydrogen gas when metal is dissolved in the solution.
2. Description of the Related Art
An etching solution and a process of this type are disclosed in DE 3,248,006.A1. One subject of this publication is an agent for the selective removal of hard metal surface coatings, particularly ceramics, cermets and mixtures based on nickel from high strength, high temperature resistant substrates. This agent is characterized by approximately 90 to 450 g/l H.sub.2 SO.sub.4, approximately 20 to 240 g/l of a water soluble, nitrosubstituted aromatic compound, the remainder water. The publication further relates to a process for removing hard metal surface coatings by employing this agent.
The water soluble, nitrosubstituted aromatic compound is preferably composed of m-nitrobenzene sodium sulfonate or m-nitrobenzene sodium sulfonic acid or the corresponding alkali or earth alkali metal nitrobenzene sulfonates; however, the other organic nitro compounds mentioned in U.S. Pat. No. 2,698,781, which will be discussed below, can also be employed.
The etching solution may additionally include hydrofluoric acid, fluoboric acid HBF.sub.4 and surfactants as wetting agents.
The publication does not reveal the manner in which the described etching solution works. The nitroaromatic compound is merely called an oxidation agent.
During practical tests with the prior art etching solution employing a sulfuric acid concentration of (i) 90 g/l, (ii) 300 g/l and (iii) 450 g/l, together in each case with 240 g/l m-nitrobenzene sodium sulfonate, it was found that hydrogen developed and was released. The etching solution was very viscous and could be employed only at an elevated temperature. Accordingly, the etching process disclosed in the publication is implemented at a temperature between about 49.degree. C. and 82.degree. C.
Another etching solution of the above-mentioned type is disclosed in the already cited U.S. Pat. No. 2,698,781. In the simplest case, this etching solution is composed of an acid and a nitrosubstituted aromatic compound whose potential under special conditions mentioned in the publication lies between 0.50 Volt and 0.90 Volt. In this etching solution, the nitrosubstituted aromatic compound is not used up during the etching process or only to a slight extent; it acts as an activator or catalyst for the etching reaction. Thus--although this is not explicitly mentioned in the publication--hydrogen has to form during the etching process. Due to the condition that the potential of the nitrosubstituted aromatic compound is to lie in the stated range, it is obvious that the compounds selected from this class of compounds are those which, after the etching process, form a reaction product neither with the metal nor with the developing hydrogen. The m-nitrobenzene sulfonic acid specially preferred in the above-mentioned DE 3,248,006.A1 is mentioned as one of the nitrosubstituted aromatic compounds that have such a potential.
Further etching solutions containing additional components are disclosed, for example in the publications CH 394,761, DE 2,635,295.C2, DE 2,536,404.C2 and DE-OS [Unexamined Published Patent Application] 2,143,785.
CH 394,761 relates to a mixture for the selective dissolution of a metal in the presence of a second metal on which the first metal is precipitated. This process is characterized in that it includes an organic nitro compound and an ammonium compound that is free from nitro groups, with the mixture being liquid and containing at least 0.01 weight % of the organic nitro compound and the ammonium compound together and having a pH value of 7 to 11. The reference further relates to a method of producing the mixture and its use for the removal of a galvanic metal coating from a metal object that is coated with it.
Aromatic nitro compounds such as, for example, the already repeatedly mentioned nitrobenzene sulfonic acid, but also nitroaliphatic compounds such as nitromethane or nitroethane may be employed as organic nitro compounds.
Although the mixture is a base, it must in no case include a strong base, such as alkali hydroxide; this is understandable because strong bases would release ammoniac from the ammonium compounds. Moreover, the concentration of hydroxyl ions should be relatively low.
DE 2,635,295.C2 discloses an etching solution for metals composed of a nitrosubstituted organic compound, a phosphorus oxyacid derivative, an organic polyamine and an added acid or base to set a pH value between 6 and 14. The metals are attacked by the nitro compound; the phosphorus oxygen derivatives form complexes of the released metal ions.
The publications DE 2,536,404.C2 and DE-OS 2,143,785 disclose even more complicated etching solutions.
These references do not indicate whether hydrogen forms from the use of these etching solutions or whether the release of hydrogen is suppressed.
The publication entitled "Fabrication of Capacitive Acceleration Sensors by the LIGA Technique" by C. Burbaum, J. Mohr, P. Bley and W. Ehrfeld, presented at the "Conference Eurosensors 4", Oct. 1-3, 1990, Karlsruhe, printed publication scheduled under the same title, discloses the use of aqueous hydrofluoric acid solution as the etching solution. The publication relates to processes for manufacturing microstructures having great structural heights in which a sacrificial layer is selectively etched away so that cantilever-type structures are created which adhere only partially to the substrate. These structures can be employed as acceleration sensors. The aqueous solution of hydrofluoric acid is employed as the etching solution. With this etching solution, a titanium layer can be selectively etched away with respect to a nickel, copper or gold layer.
As described, for example, in the last-mentioned publication, etching solutions are frequently employed in the microstructuring art in order to selectively etch away thin metal layers from other metals and creating, for example, cantilever-type or complex shaped filigree microstructures whose size often lies in a range below one hundred micrometers, approximately in a range between 5 and 10 .mu.m. It is of course understood that such microstructures may be mechanically very sensitive. In order for the thin layers to be etched away, which are often disposed at the bottom of microrecesses, to be wetted by the etching solution, the solutions employed must have a very low viscosity.
Hydrogen develops at least if etching solutions of the above-mentioned type are employed. The formation of hydrogen in the treatment of microstructured substrates is a drawback for several reasons. The formation of gas bubbles may lead to mechanical damage or deformation of the filigree microstructures. In addition, it is often necessary to optically follow the etching process very precisely and to monitor it. However, the development of hydrogen at least impedes the ability to observe, often it even makes it impossible.
It is an object of the invention to propose etching solutions and etching processes of the above-mentioned type which contain an acid or a strong base that attacks the metal and dissolves it by forming the corresponding metal ions, in which, however, the development of hydrogen is prevented or at least considerably reduced and which can therefore also be employed in the microstructuring art. In this case, however, the selective effect of the attacking agent in the etching solution should not be impaired.